Semi-dry flue gas treatment system for denitration by using ammonium sulfite generated by urea desulfurization

Abstract

The invention belongs to the technical field of flue gas purification, and particularly relates to a semi-dry flue gas treatment system for denitration by using ammonium sulfite generated by urea desulfurization. The system at least comprises a mixer and a bag-type dust collector which are communicated in sequence, the gasifier is communicated with the mixer; and the urea desulfurization system is communicated with the gasifier. According to the invention, a cloth bag is used as a reactor, and ammonium sulfite generated by urea desulfurization is subjected to denitration semi-dry flue gas treatment. Based on the system and the method, the dedusted original flue gas is discharged after being denitrated and desulfurized, ammonium sulfite is generated through urea desulfurization reaction, and the ammonium sulfite is further utilized for denitration.

Description

Technical Field

[0001] The invention belongs to the technical field of flue gas purification, and in particular relates to a semi-dry flue gas treatment system for denitrification using ammonium sulfite generated by urea desulfurization. Background Art

[0002] The flue gas generated in kilns such as steel, cement, building materials, glass, and metallurgy contains a large amount of pollutants such as sulfur dioxide, nitrogen oxides, and smoke, which cause great harm to human health and the ecological environment. The traditional treatment method is to set up an independent disposal device for a certain pollutant. To treat all pollutants, the process is long, the equipment is complex, and the investment and operating costs are high. Summary of the invention

[0003] In order to solve the deficiencies of the prior art, the present invention provides a semi-dry flue gas treatment system and method for denitrification using ammonium sulfite generated by urea desulfurization. The present invention uses a bag as a reactor to treat the semi-dry flue gas for denitrification using ammonium sulfite generated by urea desulfurization. Based on the present invention, the original flue gas after dust removal is discharged after denitrification and desulfurization, and the ammonium sulfite generated by the urea desulfurization reaction is further used for denitrification.

[0004] The technical solution provided by the present invention is as follows:

[0005] A semi-dry flue gas treatment system for denitrification using ammonium sulfite generated by urea desulfurization, comprising at least:

[0006] The mixer and the bag dust collector are connected in sequence;

[0007] a gasifier connected to the mixer;

[0008] and a urea desulfurization system connected to the gasifier.

[0009] Among the above mentioned technologies:

[0010] Ammonium sulfite is gasified in a gasifier;

[0011] The gasified ammonium sulfite enters the mixer and is fully mixed with the original flue gas after dust removal to produce ammonium sulfate particles, which enter the bag together with the flue gas. At the same time, the unreacted ammonium sulfite condenses into particles and continues to react with nitrogen oxides in the flue gas. The generated ammonium sulfate particles are attached to the surface of the bag. When the pressure difference of the bag increases, the bag is back-blown and discharged;

[0012] The flue gas passing through the bag filter enters the desulfurization tower and is further desulfurized using urea;

[0013] The ammonium sulfite generated by desulfurization can enter the gasifier and participate in the denitrification reaction;

[0014] The ammonium sulfate particles generated in the bag enter the gasifier and then enter the crystallizer from the bottom of the gasifier to achieve solid-liquid separation.

[0015] The main reaction equation of denitrification between the mixer and the bag filter is:

[0016] 2(NH4)2SO3+2NO=2(NH4)2SO4+N2

[0017] 4(NH4)2SO3+2NO2=4(NH4)2SO4+N2

[0018] The main reaction equation in the desulfurization absorption tower:

[0019] CO(NH)2+2H2O=(NH4)2CO3

[0020] (NH4)2CO3+SO2=(NH4)2SO3+CO2

[0021] (NH4)2SO3+H2O+SO2=2NH4HSO3

[0022] 2NH4HSO3+(NH4)2CO3=2(NH4)2SO3+CO2+H2O

[0023] The vaporizer utilizes the property of ammonium sulfite that it can be vaporized and that it is soluble in water but insoluble in organic solvents, and uses high-temperature heat transfer oil as the heat medium to vaporize the ammonium sulfite. At the same time, ammonium sulfate is also insoluble in heat transfer oil and will not decompose, but ammonium sulfate will be deposited in the heat transfer oil. Heat transfer oil can be used to separate ammonium sulfite and ammonium sulfate, thereby improving the utilization rate of ammonium sulfite and the purity of ammonium sulfate.

[0024] The gasifier can use external heating. To reduce the decomposition of local overheating of the heat transfer oil, the heater can use high-pressure steam heating. At the same time, the heater is matched with a circulating pump to heat the upper heat transfer oil in the gasifier to 120-160°C. The lower heat transfer oil can be left unheated and heated by heat conduction or sinking ammonium sulfate to ensure that the lower heat transfer oil is at 80-90°C. A stirrer is installed in the gasifier. The stirrer adopts a straight blade type to strengthen the radial flow of the heat transfer oil on the surface of the gasifier, ensuring that the material entering the gasifier falls evenly on the surface of the heat transfer oil. The solid ammonium sulfate particles wrapped in the heat transfer oil then spirally sink along the wall of the gasifier to the bottom under the action of centrifugal force and enter the oil-water separation chamber.

[0025] The gasifier adopts an inverted cone bottom, which can collect the sinking ammonium sulfate and then enter the oil-water separation chamber. Taking advantage of the immiscible nature of heat transfer oil and water, the ammonium sulfate particles in the oil-water separation chamber fall from the heat transfer oil side to the aqueous solution side and dissolve. The upper part of the oil-water separation chamber is heat transfer oil and the lower part is aqueous solution. Through the glass window, the oil-water interface can be clearly seen. Two layers of screens are installed on the water side of the lower half. The aperture near the oil side is larger, for example, 5mm, to intercept the large ammonium sulfate particles wrapped in heat transfer oil, and as the ammonium sulfate particles gradually dissolve, the oil droplets become smaller and float up; the lower aperture is smaller, for example, 0.2mm, to capture small particles of oil droplets mixed with ammonium sulfate in the aqueous solution. A device for cleaning oil droplets is set on the filter, such as a scraper device with a brush. When there are more oil droplets attached to the filter, the scraper device is started to peel off the small oil droplets from the filter surface and then float to the oil-water interface. The heat transfer oil adopts a higher boiling point and more stable alkyl biphenyl type. The oil-water separation chamber of the vaporizer is connected to the crystallizer through a pipeline, and the dissolved ammonium sulfate is sent to the crystallizer through the bottom of the separation chamber through the circulating solution. The vaporizer should be sealed to prevent the vaporized ammonium sulfite from leaking out and polluting the environment. At the same time, the vaporizer and heater should be installed with insulation, and the ammonium sulfite gasification pipeline should be installed with heating to ensure that the ammonium sulfite vapor is not lower than 100℃.

[0026] The bag filter is used to collect solid particles of ammonium sulfite and sulfuric acid in the flue gas. At the same time, the solid particles of ammonium sulfite are adsorbed on the surface of the bag and continue to react with the remaining nitrogen oxides in the flue gas to generate ammonium sulfate particles. The mixer at the inlet of the bag filter is used to fully mix the gasified ammonium sulfite with the flue gas, which can be a combination of multi-point distribution and swirl.

[0027] Specifically, the urea desulfurization system comprises:

[0028] An induced draft fan connected to the bag filter;

[0029] A desulfurization tower connected to the induced draft fan;

[0030] A urea dissolving tank connected to the desulfurization tower, wherein the desulfurization tower has a liquid outlet;

[0031] and a centrifugal separator, which is respectively connected to the liquid outlet of the desulfurization tower and the gasifier.

[0032] The urea desulfurization system includes a desulfurization tower, a urea dissolving tank, a centrifuge, etc.

[0033] The desulfurization tower can be an absorption tower, which adopts gas-liquid countercurrent contact. The flue gas flows from bottom to top, and the absorption solution flows downward from the top of the tower and is dispersed in the gas phase in the form of droplets. In the absorption tower: the gas and liquid have sufficient contact time and area; the gas-liquid two phases can be strongly disturbed, the operating range is wide, the operation is stable, and the resistance is small; it has sufficient mechanical strength and corrosion resistance.

[0034] The bottom of the desulfurization tower is connected to the circulation pump and the liquid discharge pump. The tower adopts multi-layer spraying. The nozzle adopts a pressure-type cone nozzle. The spraying angle is large, uniform and not easy to be blocked. The absorption liquid is sent to the nozzle through the circulation pump. The demister is arranged at the top of the tower. An integrated tubular dust removal and demister can be used to reduce the droplet carryover and reduce the sulfate content in the clean flue gas. The demister needs to be flushed with process water regularly. The flow rate is adjusted by the urea supplementary liquid pump to control the pH value of the desulfurization tower solution between 6-7.

[0035] The urea dissolving tank is equipped with 30-40% urea solution.

[0036] As the desulfurization reaction continues, the concentration of ammonium sulfite in the desulfurization tower not only increases, but when the ammonium sulfite in the desulfurization tower reaches supersaturation and the solid content is 2-10%, the slurry can be further sent to the cyclone through the discharge pump for preliminary separation, and then enter the rotary drum centrifuge for liquid-solid separation, and the filtrate returns to the desulfurization tower for recrystallization and growth.

[0037] Specific:

[0038] The desulfurization tower is provided with an absorption liquid spraying device;

[0039] The absorption liquid spraying device is connected to the urea dissolving tank.

[0040] Specifically, the desulfurization tower has an exhaust port, and the exhaust port of the desulfurization tower is connected to the chimney.

[0041] Specific:

[0042] The bottom of the bag dust collector is provided with a dust outlet;

[0043] The dust outlet is connected to the gasifier.

[0044] Specific:

[0045] The gasifier is provided with a stirrer;

[0046] The gasifier adopts an inverted cone bottom.

[0047] Further:

[0048] The bottom of the gasifier is connected to an oil-water separation chamber;

[0049] The upper part of the oil-water separation chamber is provided with an oil-water separation zone;

[0050] A large oil droplet catching net is arranged in the middle of the oil-water separation chamber;

[0051] A small oil droplet catching net is arranged at the lower part of the oil-water separation chamber.

[0052] Furthermore: the oil-water separation chamber is connected to the crystallizer through a circulation pipeline, and the crystallizer is connected to the centrifuge.

[0053] The crystallizer can be in the form of a guide tube, and the agitator adopts axial flow multi-stage blades. The size of the crystal particles is controlled according to the speed of the agitator. The crystallizer is equipped with a steam heater and a circulation pump to ensure that the water side temperature of the oil-water separation chamber is 80-90°C, which promotes the rapid dissolution of ammonium sulfate. When ammonium sulfate continuously enters the crystallizer and reaches supersaturation, it begins to crystallize. After adjusting the agitator speed to make the crystal particles grow, they are sent to the cyclone for preliminary separation through the discharge pump and then enter the rotary drum centrifuge for liquid-solid separation. The filtrate returns to the crystallizer for recrystallization and growth.

[0054] Specific:

[0055] The bag filter includes a plurality of filter units;

[0056] The inlet of each of the dust collector units is connected to the smoke inlet pipe;

[0057] The outlet of each dust collector unit is connected to the smoke outlet pipe;

[0058] The back-blowing port of each dust collector unit is connected to a back-blowing air fan;

[0059] Each two adjacent dust collector units are connected via a back-blowing flow port.

[0060] In the above technical method, the bag filter is composed of several units. In order to reduce the impact of bag backflushing on the denitrification of the system, the number of units is preferably greater than 10. Each unit is composed of several bags, and the inlet of each unit is connected, and the outlet can be isolated separately, which is used to isolate the running bags from each other during bag backflushing. There is a large-angle hopper and a star valve under each bag unit for storage and discharge. The bag adopts a retractable spring frame with large deformation, which is easy to peel off the particles attached to the surface of the bag. The bag filter is equipped with a large bypass, which is opened when the system starts and closed when the system temperature rises.

[0061] The bags can be made of polyester or polypropylene fiber bags to prevent acid corrosion, temperature resistance at 150℃, and have the characteristics of low water absorption and strong air permeability. Use long bags with a diameter of 200-300mm and a length of 8-10m to reduce the volume of the bag filter and ensure that the bag filter air velocity is 0.5-1m / min.

[0062] Specifically, the mixer is connected to the original flue gas pipe after dust removal.

[0063] Furthermore, the mixer is connected to the bag filter through a flue, and sulfur dioxide and nitrogen oxide analyzers, as well as temperature, pressure and air volume meters are installed at the inlet and outlet of the bag filter.

[0064] Furthermore, the mixer is connected to the outlet of the gasifier, and the material outlet of the gasifier is connected to the crystallizer through a circulation pipeline.

[0065] Furthermore, the bag filter outlet is connected to the induced draft fan inlet through a pipeline, and a temperature and pressure gauge is installed in the middle. The bag filter outlet is connected to the gasifier feed inlet.

[0066] Furthermore, the outlet of the induced draft fan is connected to the inlet pipe of the desulfurization tower through a flue, and a temperature and pressure gauge is installed in the middle. A sulfur dioxide and nitrogen oxide analyzer, as well as a temperature, pressure and air volume gauge are installed at the outlet of the desulfurization tower.

[0067] Furthermore, the outlet of the desulfurization tower is connected to the chimney through a flue.

[0068] Furthermore, the desulfurization tower is connected to a rotary drum centrifuge through a liquid outlet pipe, and a discharge pipe of the centrifuge is connected to a gasifier.

[0069] Furthermore, the heater of the vaporizer and the heater of the crystallizer are connected to the steam pipeline through pipelines, and the steam can be firstly used to heat the vaporizer and then added to the crystallizer, thereby improving the steam utilization rate.

[0070] The present invention also provides a semi-dry flue gas treatment method for denitrification using ammonium sulfite generated by urea desulfurization, comprising the following steps:

[0071] The ammonium sulfite vaporized in the gasifier enters the mixer and is fully mixed with the original flue gas after dust removal to react and generate ammonium sulfate particles. The ammonium sulfate particles enter the bag filter together with the flue gas.

[0072] Unreacted ammonium sulfite condenses into particles and reacts with nitrogen oxides in the original flue gas after dust removal to generate ammonium sulfate particles, which are attached to the surface of the bag filter;

[0073] In the urea desulfurization system, urea is used to desulfurize the flue gas discharged from the bag filter, and the ammonium sulfite generated by desulfurization enters the gasifier.

[0074] Preferably, the upper layer of heat transfer oil in the gasifier is heated to 120-160°C.

[0075] Preferably, the lower layer of heat transfer oil is heated by heat conduction or sinking ammonium sulfate to ensure that the temperature is 80-90°C.

[0076] The beneficial effects of the present invention are as follows:

[0077] Compared with traditional desulfurization and denitrification, the present invention does not use a catalyst during denitrification, does not generate wastewater, is beneficial to environmental protection and safety, and the generated ammonium sulfate can be used in other industries. Compared with wet desulfurization, it has higher efficiency and the equipment is not easy to be blocked. BRIEF DESCRIPTION OF THE DRAWINGS

[0078] Figure 1 It is a structural schematic diagram of a semi-dry flue gas treatment system for denitrification using ammonium sulfite generated by urea desulfurization provided by the present invention.

[0079] Figure 2 It is the schematic diagram of the oil-water separation chamber.

[0080] Figure 3 It is a structural diagram of a bag dust collector.

[0081] Attached Figure 1 , 2 , 3, the structure list represented by each number is as follows:

[0082] 1. Gasifier; 2. Crystallizer; 3. Mixer; 4. Bag filter; 41. Dust collector unit; 42. Flue gas inlet pipe; 43. Flue gas outlet pipe; 44. Back-blowing air fan; 45. Back-blowing flow port; 5. Induced draft fan; 6. Centrifuge; 7. Desulfurization tower; 8. Urea dissolving tank; 9. Centrifuge; 10. Glass window. DETAILED DESCRIPTION

[0083] The principles and features of the present invention are described below. The examples given are only used to explain the present invention and are not used to limit the scope of the present invention.

[0084] Unless otherwise specified, the test methods used in the examples are conventional methods; the materials and reagents used are commercially available reagents and materials unless otherwise specified.

[0085] Example 1

[0086] like Figure 1 As shown, the semi-dry flue gas treatment system for denitrification using ammonium sulfite generated by urea desulfurization includes: a mixer 3, a bag filter 4 and a gasifier 1. The mixer 3 and the bag filter 4 are connected in sequence. The gasifier 1 is connected to the mixer 3. The urea desulfurization system is connected to the gasifier 1.

[0087] A certain liquid level of heat transfer oil is maintained in the gasifier, and the heat transfer oil is heated to 120-160°C. After the ammonium sulfate and ammonium sulfite particles enter the gasifier, part of the ammonium sulfite is vaporized on the surface of the high-temperature heat transfer oil, and the other part is decomposed into ammonia and sulfur dioxide, which enter the flue gas mixer together for denitrification reaction. The ammonium sulfate sinks into the oil-water separation chamber at the bottom of the gasifier and dissolves, realizing the separation of ammonium sulfate and ammonium sulfite at the same time.

[0088] The vaporized ammonium sulfite reacts with nitrogen oxides in the flue gas to undergo a denitrification reaction to generate solid ammonium sulfate particles and crystalline particles of vaporized ammonium sulfite, which enter the bag filter together with the flue gas. The solid ammonium sulfite particles are adsorbed on the surface of the bags and continue to react with nitrogen oxides in the flue gas passing through the bags to generate ammonium sulfate particles. When the pressure difference of the bags increases, the bags are back-blown and discharged.

[0089] In the urea desulfurization system, desulfurization is carried out by urea solution. When the generated ammonium sulfite solution reaches supersaturation and the solid content reaches 2-10%, it can be further sent to a centrifuge for liquid-solid separation. The generated ammonium sulfite can enter the gasifier for denitrification.

[0090] Example 2

[0091] On the basis of Example 1, Figure 1 , 3 As shown, the bag filter includes a plurality of dust collector units 41. The inlet of each dust collector unit 41 is connected to the flue gas inlet pipe 42. The outlet of each dust collector unit 41 is connected to the flue gas outlet pipe 43. The back-blowing port of each dust collector unit 41 is connected to the back-blowing air fan 44. Each two adjacent dust collector units 41 are connected through the back-blowing flow port 45. The bottom of the bag filter 4 has a dust outlet. The dust outlet is connected to the gasifier 1. Based on this technical solution, in the bag filter, the solid particles of ammonium sulfite are adsorbed on the surface of the bag, and continue to react with the nitrogen oxides in the flue gas passing through the bag to generate ammonium sulfate particles. When the pressure difference of the bag increases, the bag is back-blown and discharged.

[0092] Example 3

[0093] On the basis of Example 1, Figure 1 As shown, the urea desulfurization system includes: an induced draft fan 5, a desulfurization tower 7, a urea dissolving tank 8, and a centrifugal separator 6. The induced draft fan 5 is connected to the bag filter 4. The desulfurization tower 7 has a liquid outlet, an absorption liquid spraying device, and an exhaust port. The desulfurization tower 7 is connected to the induced draft fan 5. The absorption liquid spraying device is connected to the urea dissolving tank 8. The exhaust port of the desulfurization tower 7 is connected to the chimney. The centrifugal separator 6 is respectively connected to the liquid outlet of the desulfurization tower 7 and the gasifier 1. Based on this technical solution, the ammonium sulfite generated by desulfurization of the urea solution can be sent to the gasifier for denitrification.

[0094] Example 4

[0095] On the basis of Example 1, Figure 1 As shown, a stirrer is provided in the gasifier 1, and an inverted cone bottom is adopted. An oil-water separation chamber is provided at the bottom of the gasifier 1. The upper part of the oil-water separation chamber has an oil-water separation area. A large oil droplet catcher net is provided in the middle part of the oil-water separation chamber. A small oil droplet catcher net is provided at the lower part of the oil-water separation chamber. A glass window 10 can be provided in the oil-water separation chamber. Figure 2 As shown, the oil-water separation interface, the large oil droplet capture layer and the small oil droplet capture layer are formed from top to bottom. Based on this technical solution, the oil-water separation chamber of the gasifier is connected to the crystallizer through a pipeline, and the dissolved ammonium sulfate can be sent to the crystallizer through the circulating solution at the bottom of the separation chamber.

[0096] Example 5

[0097] On the basis of Example 1, Figure 1 As shown, the oil-water separation chamber is connected to the crystallizer 2 through a circulation pipeline, and the crystallizer 2 is connected to the centrifuge 9. Ammonium sulfate particles in the gasifier continuously enter the crystallizer. When the ammonium sulfate solution reaches supersaturation, the ammonium sulfate particles in the slurry continue to grow in the crystallizer, and then liquid-solid separation can be performed.

[0098] Effect Example 1

[0099] The semi-dry flue gas treatment method using ammonium sulfite denitrification generated by urea desulfurization is implemented in the following steps:

[0100] 1. After the system is ready, prepare 30-40% urea solution in the urea dissolving tank, send a certain level of urea solution into the desulfurization tower to ensure that the liquid level is sufficient for circulation, then start the desulfurization circulation pump to start the absorption liquid circulation.

[0101] 2. Start the induced draft fan to draw in smoke to heat up the system, open the bag filter bypass valve, and adjust the system air volume and pressure to ensure that the negative pressure at the bag filter inlet is -0.5--2KPa. When the bag filter outlet temperature reaches 90℃, close the bag filter bypass valve and heat up the bag filter.

[0102] 3. When the pH value of the absorption liquid in the desulfurization tower is 6-7 and the temperature is 60-70℃, when the ammonium sulfite solution in the tower is supersaturated, crystal particles will precipitate. When the solid content reaches 10%, the desulfurization tower outlet pump is started to send slurry to the cyclone. The supernatant of the cyclone returns to the desulfurization tower, and the slurry with high solid content in the lower part enters the centrifuge. After separation, the solid is sent to the gasifier for denitrification, and the filtrate is returned to the desulfurization tower for recrystallization.

[0103] 4. Start the agitator and steam heater in the gasifier to raise the temperature of the heat transfer oil to between 140-160℃, adjust the quality of ammonium sulfite entering the gasifier, and adjust the nitrogen oxides at the outlet of the bag filter. Generally, the sulfur dioxide in flue gas is much higher than the nitrogen oxides, so the desulfurization product ammonium sulfite is more, which is more than what is needed for denitrification, so the ammonium sulfite produced by desulfurization is excessive; because the ammonium sulfite from the centrifuge has a low content and a high moisture content, generally about 80% of the effective ingredients, the vaporizer adds ammonium sulfite with a high moisture content, and the evaporation of the moisture requires more steam, so the steam input needs to be increased.

[0104] 5. At the same time, according to the bag pressure difference, when the bag pressure difference is 1.0-2.0KPa, start the bag dust collector backflush system, and backflush each dust collector unit in turn. When backflush, close the outlet valve of the dust collector unit, open the backflush air inlet valve of the dust collector unit, and start the bag backflush air fan. The backflush time of each dust collector unit is determined to be 20-40s. When the backflush of the dust collector unit is completed, close the backflush air inlet valve of the dust collector unit and open the outlet valve of the dust collector unit. Backflush the next unit, and stop the backflush air fan after all backflush are completed. At the same time, start the hopper star valve for unloading.

[0105] 6. The material collected by the bag filter is sent to the gasifier. Since the material collected by the bag filter contains ammonium sulfite and ammonium sulfate, and has a low moisture content, less steam is put into the gasification. The mixed material of ammonium sulfate and ammonium sulfite sent to the gasifier is separated and reused. Ammonium sulfate enters the oil-water separation chamber through the bottom of the gasifier and dissolves on the water side. At the same time, the delivery amount of ammonium sulfite from the centrifuge outlet to the gasifier is adjusted.

[0106] 7. Start the agitator, heater and circulation pump of the crystallizer, and send the dissolved ammonium sulfate on the water side of the oil-water separation chamber into the crystallizer. When the ammonium sulfate is continuously sent from the gasifier to the crystallizer, the ammonium sulfate particles in the recrystallizer continue to grow. When the solid content of the slurry reaches 10%, start the discharge pump to send the slurry to the centrifuge for solid-liquid separation, and the filtrate returns to the crystallizer. The collected ammonium sulfate can be dried, packaged and shipped out.

[0107] The parameters are as follows:

[0108] The urea dissolving tank is filled with 35% urea solution.

[0109] The heat transfer oil is heated to about 140°C (usually 120-160°C).

[0110] The components of the original flue gas after dust removal are: nitric oxide: 200mg / m; sulfur dioxide 3000mg.

[0111] The chimney exhaust meets the emission standards, with low concentrations of nitrogen monoxide and sulfur dioxide: less than 20 mg respectively.

[0112] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A semi-dry flue gas treatment system for denitrification using ammonium sulfite generated by urea desulfurization, characterized in that: At least: A mixer (3) and a bag filter (4) are sequentially connected; A gasifier (1) connected to the mixer (3); and a urea desulfurization system, which is connected to the gasifier (1).

2. The semi-dry flue gas treatment system for denitrification using ammonium sulfite generated by urea desulfurization according to claim 1 is characterized in that: The urea desulfurization system comprises: An induced draft fan (5) connected to the bag filter (4); A desulfurization tower (7), which is connected to the induced draft fan (5); A urea dissolving tank (8) connected to the desulfurization tower (7), wherein the desulfurization tower (7) has a liquid outlet; and a centrifugal separator (6), which is respectively connected to the liquid outlet of the desulfurization tower (7) and the gasifier (1).

3. The semi-dry flue gas treatment system for denitrification using ammonium sulfite generated by urea desulfurization according to claim 2 is characterized in that: The desulfurization tower (7) has an absorption liquid spraying device; The absorption liquid spraying device is connected to the urea dissolving tank (8).

4. The semi-dry flue gas treatment system for denitrification using ammonium sulfite generated by urea desulfurization according to claim 2 is characterized in that: The desulfurization tower (7) has an exhaust port; The exhaust port of the desulfurization tower (7) is connected to the chimney.

5. The semi-dry flue gas treatment system for denitrification using ammonium sulfite generated by urea desulfurization according to claim 1 is characterized in that: The bag dust collector (4) has a dust outlet at the bottom; The dust outlet is connected to the gasifier (1).

6. The semi-dry flue gas treatment system for denitrification using ammonium sulfite generated by urea desulfurization according to claim 1, characterized in that: The gasifier (1) is provided with a stirrer; The gasifier (1) has an inverted cone-shaped bottom.

7. The semi-dry flue gas treatment system for denitrification using ammonium sulfite generated by urea desulfurization according to claim 6 is characterized in that: The bottom of the gasifier (1) is connected to an oil-water separation chamber; The upper part of the oil-water separation chamber is provided with an oil-water separation zone; A large oil droplet catching net is arranged in the middle of the oil-water separation chamber; A small oil droplet catching net is arranged at the lower part of the oil-water separation chamber.

8. The semi-dry flue gas treatment system for denitrification using ammonium sulfite generated by urea desulfurization according to claim 7 is characterized in that: The oil-water separation chamber is connected to a crystallizer (2) through a circulation pipeline, and the crystallizer (2) is connected to a centrifuge (9).

9. The semi-dry flue gas treatment system for denitrification using ammonium sulfite generated by urea desulfurization according to any one of claims 1 to 8, characterized in that: The bag filter (4) comprises a plurality of filter units (41); The inlet of each dust collector unit (41) is connected to the smoke inlet pipe (42); The outlet of each dust collector unit (41) is connected to a smoke outlet pipe (43); The back-blowing port of each dust collector unit (41) is connected to a back-blowing air fan (44); Each two adjacent dust collector units (41) are connected via a back-blowing flow port (45).

10. A semi-dry flue gas treatment method using ammonium sulfite generated by urea desulfurization for denitrification, characterized in that: The processing system according to any one of claims 1 to 9 is used for processing, comprising the following steps: The ammonium sulfite gasified in the gasifier (1) enters the mixer (3), is fully mixed with the original flue gas after dust removal, and reacts to generate ammonium sulfide particles. The ammonium sulfide particles enter the bag filter (4) together with the flue gas. Unreacted ammonium sulfite condenses into fine particles and reacts with nitrogen oxides in the original flue gas after dust removal to generate ammonium sulfate particles, which are attached to the surface of the bag filter (4); In the urea desulfurization system, urea is used to desulfurize the flue gas discharged from the bag filter (4), and the ammonium sulfite generated by the desulfurization enters the gasifier (1).

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